One exhaust structure for a small watercraft is disclosed in Japanese Patent Laid-open No. 2003-176719. This exhaust structure includes an exhaust system for discharging the exhaust gas of an engine to the outside. A water muffler is provided in the middle of the exhaust system. An exhaust body (backflow prevention chamber) adapted to prevent water from entering toward the engine from the water muffler is disposed above the water muffler. In addition, a water jacket is provided in the circumferential wall of the backflow prevention chamber.
When causing exhaust gas from the engine to flow in the exhaust system and discharge it to the outside, such an exhaust structure for a small watercraft can cool the exhaust gas in the backflow prevention chamber by allowing cooling water to flow in the water jacket.
However, the exhaust structure of Japanese Patent Laid-open No. 2003-176719 is provided with the water jacket serving as a cooling water passage, in the circumferential wall of the backflow prevention chamber. It is necessary, therefore, to relatively increase the thickness of the circumferential wall of the backflow prevention chamber. This hinders the downsizing and weight saving of the backflow prevention chamber. In addition, the provision of the water jacket in the circumferential wall of the backflow prevention chamber complicates the shape of the chamber, which hinders the decrease in the cost of the chamber.